Hi Tui, I have two large lemon trees at least 35 years old that normally produce heavy crops. Over the past year the leaves have started to go black and also the skins of some fruit. I regularly spray for bugs and also copper. What else can I do to get rid of whatever is causing the blackening?
This is sooty mould which is a fungus that lives on the honey dew secreted from sucking insects such as scale, aphids or mealy bug. Can you see any insects on the tree? Maybe bumps on the stems and under side of leaves? This could be scale insect. Once you get rid of the insects, the sooty mould will go away. Spray milk on the tree to get rid of the sooty mould, it washes off in the rain eventually, but will return if you don’t control the insects that are affecting your tree. Ask at your local garden centre or DIY store for a suitable control for sucking insects. An application of spraying oil will help get rid of the insects and also the sooty mould. Be sure to alternate sprays and not use the same one all the time as insects can build up resistance. The black does not affect the fruit, it just looks unsightly and will wash off easily.
Thanks, Tui Team